Nonlinear interactions of internal gravity waves in a continuously stratified fluid

Abstract

Internal gravity waves are buoyancy-driven oscillations which can arise in a density-stratified fluid. They exist throughout the oceans and atmosphere, the oceanic internal wavefield being sufficiently energetic for nonlinear effects to play an important role in the internal wave dynamics. Numerical studies of oceanic internal waves (such as Broutman & Young, 1986) have suggested that under the right conditions a weak internal gravity wave can be strongly refracted and frequency-shifted by the time-varying shear of a large-amplitude internal wave. My project aimed to experimentally observe this type of strongly nonlinear interaction by generating the required internal gravity waves in a continuously stratified aqueous solution. The waves were observed using a colour schlieren system, and power spectra of the internal wavefield were obtained using conductivity probes and polarimetry. Several nonlinear phenomena were observed, including anharmonic waves and forced sum and difference frequencies, as well as second-harmonic generation from the wave sources. However a combination of wavelength limitations imposed by viscosity, inescapable restrictions on the strong wave amplitude and severe observational difficulties all conspired to prevent detection of the particular nonlinear interaction of interest. A proposed apparatus could overcome these difficulties, but its construction would be quite beyond the scope of an Honours project. Show more